Abstract : The pyrolytic decomposition of hexanitrohexazaisowurtzitane (HNIW) and a HNIW-based propellant formulation were investigated using several infrared and mass spectrometric techniques. The pyrolysis produce distributions of HNIW (500 C and 1,000 C pyrolysis) were compared to those of HMX and RDX. At both temperatures, HNIW produced a higher CO2N2O ratio than did HMX or RDX. Several products were generated by all three nitramines, while others were unique to HNIW. When heated under vacuum conditions, HMX appears to undergo both vaporization and decomposition, while HNIW appears to remain in the condensed phased prior to rapid decomposition. Investigation of the effect of propellant ingredients (i.e., nitrate ester, plasticizers and a modified DuPont HYCAR thermoplastic elastomer) on the pyrolysis product distribution of a HNIW-based propellant indicates the removal of several HNIW decomposition products by those ingredients. Observed variations in the pyrolysis product distribution as a function of sample size are attributed to as yet unidentified secondary reactions.